Effect of La addition on adhesive strength and fracture behavior of Sn–3.5Ag solder joints

This study investigates the effects of trace amounts of La addition on the microstructure, adhesive strength and fracture behavior of Sn–3.5Ag solder joints. The electron backscatter diffraction (EBSD) results confirmed that the face-centered cubic (fcc) structure of LaSn3 compounds distributed randomly in the β-Sn matrix had no obvious preferred orientation. La additions reduced the ripening reaction rate of the scallop-type Cu6Sn5 compounds at the interface layer. In spite of the significant refinement of microstructure and suppression of the growth of intermetallic Cu6Sn5 compounds caused by the La additions, the expected enhancement of adhesive strength and ductility did not occur apparently. With the aid of Argon Beam Milling, we have found defects and voids inside the LaSn3 compounds; these features were closely related to the crack initiation and formation during the adhesive tensile test. Consequently, the beneficial effect due to the refinement of microstructure and suppression of interfacial layer compounds was retarded. In the aged samples, trace amounts of La addition suppressed the Sn diffusion and retarded the growth of the Cu6Sn5 layer, but not that of the Cu3Sn layer.